Linear actuators are used in various systems carried onboard a satellite, such as for orienting propulsion devices devoted to the change of orbit or for keeping a satellite in position in its orbit for example. In these systems an electric motor transmits a rotational movement to a screw-nut mechanism that converts the rotational movement into a translational movement. In order to improve the conversion efficiency or when a high degree of precision is required in the movement, mechanisms of the roller screw type may be used. Numerous linear actuators use satellite roller screw mechanisms, particularly on account of their long life and compactness.
FIG. 1 depicts a linear actuator comprising a roller screw mechanism. In the known way, the roller screw mechanism 10 comprises rollers 9 interposed between an endless screw 11 and two tapped rings 12a and 12b connected to a housing 13. The housing is guided in rotation with respect to the structure 14 by means of a rolling bearing 15. In the example depicted, the housing is rotationally driven by a rotary motor 16. The endless screw 11 is connected to the structure in rotation and free in translation. The rotational movement of the housing 13 allows the endless screw 11 to be driven translationally with respect to the structure. The principle of roller screw mechanisms is known and is not referred to here, particularly the means that allow the rollers to be returned to a central position with respect to the two tapped rings at the end of each turn of the endless screw.
There are various preloading devices for roller screw mechanisms. These devices, which apply a preload to the mechanism in order to take up lash within the mechanism are commonly employed in the field of aerospace. When the mechanism is required to be subjected to high levels of vibration and to operate in a strong vacuum, as is notably the case during the launch of a spacecraft, the repeated relative axial movements between the housing, the rollers and the tapped rings generate repeated impacts which may damage the elements of the mechanism or cause them to cease. In one known embodiment, an axial preload is applied between the two tapped rings, so as to keep the moving parts in contact with one another. Various mechanisms capable of applying a compressive preload between the two tapped rings are known.
Known in particular is the preloading device described by the patent application published under the reference FR2699633, the principle of which is depicted in FIG. 1. The two tapped rings 12a and 12b are mounted in the housing 13 in such a way that a first tapped ring, here referenced 12a, is secured to the housing, and a second tapped ring, here referenced 12b, is connected in rotation to the housing and free in translation. The preloading device is a spring mechanism 17 comprising a compression coil spring 18 placed around the endless screw and between two bearing rings 19a and 19b. The bearing ring 19a is in contact with the tapped ring 12b. The bearing ring 19b is in contact with a preloading nut 20 mounted on the housing so that a compressive force can be applied to the coil spring.
The linear actuator consists of the roller screw mechanism 10 guided in rotation by the rolling bearing 15. Rotational guidance is also exposed to high levels of vibration and to the strong vacuum conditions. For this reason it too comprises a preloading device. In the example depicted in FIG. 1, the rolling bearing consists of hard preload rolling bearings of the super duplex type. The bearing 15 comprises two rows of angular-contact ball bearings mounted in the DF configuration. The inner rings 21a and 21b of the two rows of bearings are loaded by means of a preloading nut 22 mounted on the structure.
Thus, the linear actuator comprises two preloading devices capable respectively of preloading the roller screw mechanism and of guiding the rotation thereof relative to the structure. This results in a linear actuator which is complex, the preloading of which requires a high number of components (bearing rings, coil spring, preloading nuts, preloaded super duplex rolling bearings, etc.). All of these components, which are essentially made of stainless steel, represent a significant mass and volume. The linear actuator is also complex to design and to manufacture. It is therefore still desirable to have available a linear actuator that incorporates a preloading device that is simple, inexpensive, and compatible with the most ambitious requirements of the field of aerospace.